In hemophilia, the clotting of blood is disturbed by a lack of certain plasma blood clotting factors. Human factor IX (FIX) is a zymogen of a serine protease that is an important component of the intrinsic pathway of the blood coagulation cascade. In individuals who do not have FIX deficiency, the average half-life of FIX is short, approximately 18-24 hours. A deficiency of functional FIX, due to an X-linked disorder that occurs in about one in 30,000 males, results in hemophilia B, also known as Christmas disease, named after a young boy named Stephen Christmas who was found to be lacking this factor. Over 100 mutations of factor IX have been described; some cause no symptoms, but many lead to a significant bleeding disorder. When untreated, hemophilia B is associated with uncontrolled bleeding into muscles, joints, and body cavities following injury, and may result in death. Previously, treatments for the disease included administration of FIX prepared from human plasma derived from donor pools, which carried attendant risks of infection with blood-borne viruses including human immunodeficiency virus (HIV) and hepatitis C virus (HCV). More recently, recombinant FIX products have become commercially available.
The in vivo activity of exogenously supplied factor IX is limited both by protein half-life and inhibitors of coagulation, including antithrombin III. Factor IX compositions typically have short half-lives, requiring frequent injections. Also, current FIX-based therapeutics requires intravenous administration due to poor bioavailability. Thus, there is a need for improved factor IX compositions with extended half-life and retention of activity when administered as part of a preventive and/or therapeutic regimen for hemophilia, including hemophilia B.
Factor VII is a coagulation factor protein synthesized in the liver and secreted into the blood as a single chain zymogen with a molecular weight of approximately 50 kDa. The FVII zymogen is converted into an activated form (FVIIa) by proteolytic cleavage, and the activated form, when complexed with tissue factor (TF), is able to convert both factor IX and factor X into their activated forms, leading to rapid thrombin generation and fibrin formation. Because the circulating half-life of rFVIIa is about 2.3 hours (“Summary Basis for Approval for NovoSeven©”, FDA reference number 96-0597), multiple and frequent administrations are required for the treatment of bleeding disorders in hemophiliacs and subjects with factor VII deficiency.
Chemical modifications to a therapeutic protein can reduce its in vivo clearance rate and subsequent increase serum half-life. One example of a common modification is the addition of a polyethylene glycol (PEG) moiety, typically coupled to the protein via an aldehyde or N-hydroxysuccinimide (NHS) group on the PEG reacting with an amine group (e.g. lysine side chain or the N-terminus). However, the conjugation step can result in the formation of heterogeneous product mixtures that need to be separated, leading to significant product loss and complexity of manufacturing and does not result in a completely chemically-uniform product. Also, the pharmacologic function of the therapetuics protein may be hampered if amino acid side chains in the vicinity of its binding site are modified by the PEGylation process. Fusing an Fc domain to the therapeutic protein is another approach to increases the size of the therapeutic protein, hence reducing the rate of clearance through the kidney. Additionally, the Fc domain confers the ability to bind to, and be recycled from lysosomes by, the FcRn receptor, which results in increased pharmacokinetic half-life. Unfortunately, the Fc domain does not fold efficiently during recombinant expression, and tends to form insoluble precipitates known as inclusion bodies. These inclusion bodies must be solubilized and functional protein must be renatured from the misfolded aggregate. Such process is time-consuming, inefficient, and expensive. Accordingly, there remains a need for improved coagulation factor compositions with increased half-life which can be administered less frequently, and/or be produced by a simpler process at a cheaper cost.